When combusting waste liquors in pulp processes, the aim is to separate the organic and the inorganic parts of the dry substance of the waste liquor from each other. The heat from the organic part of the dry substance is recovered and the largest possible amount of steam is produced by means of this heat. Pulping chemicals are recovered from the inorganic part of the dry substance in such a form that they can, in subsequent stages of processing, be converted into a suitable form to be reused in the cooking process.
The soda recovery boiler has, until now, proved to be superior for the recovery of heat and chemicals from waste liquors. The waste liquor is sprayed in the form of small drops into the boiler. In the hot combustion chamber water vapor, volatile parts of the dry substance, and eventually gasifiable parts of the dry substance evaporate from the drops. The gases inflame, thereby delivering heat to the heat surfaces disposed in the boiler and are discharged from the upper end of the boiler. The ash from the waste liquor drops, i.e. the inorganic substances of the waste liquor, accumulate on the bottom of the boiler, from which they are removed and through various stages of processing are conveyed back to the cooking process.
The flue gases from the soda recovery boiler contain a great deal of ash, mainly sodium sulfate, a portion of which flows along with the flue gases upwards in the boiler in the form of fine dust or molten drops. The salts contained in the ash melt at a relatively low temperature and become, when melting, easily adhesive and corrosive. The deposits formed by the molten ash cause the risk of clogging of the flue gas channels and, furthermore, cause corrosion and erosion of the heat surfaces of the boiler. The risk of clogging and corrosion increases considerably the number of the shutdowns for inspection and maintenance.
Salt corrodes metal, particularly if the salt is molten or partly molten. A high temperature of the boiler tubes speeds up the formation of deposits and thereby the corrosion of the heat surfaces. Thus, the deposits affect particularly the heat surfaces for the superheated steam. Usually the corrosion of the materials is reduced by controlling the temperature of the superheater surfaces.
In the places of the superheater, in which the temperature tends to rise especially high or in which there is a great deal of liquid-phase chemicals, in other words, where the corrosion and the erosion are a problem, special-alloy steels have to be used. Special-alloy steels are, however, remarkably more expensive than carbon steels or pressure vessel steels, which are commonly used, such as chrome/molybdenum-alloy steels. Even special-alloy steels have their maximum operating temperatures, above which they behave in the same way as the cheaper pressure vessel steels. This temperature is substantially lower in soda recovery boilers than in, for instance, oil-fired boilers. Further, connecting special-alloy steel by welding to carbon steel requires special circumstances, e.g. shielding gas, superalloyed filler metals and a demanding welding technique.
If the endurance of the Superheater can be improved material costs are saved and the utilization rate of the pulp mill is improved due to the reduced need for shutdowns for maintenance.
Today, the principal way of avoiding corrosion is to choose a sufficiently low temperature and pressure for the produced steam, whereby the detrimental effects of the molten salt decrease. This means that the steam cannot be superheated to as high a temperature as desired for the maximal production of electric power in steam turbine plants.
In a steam power plant, the higher the pressure and the temperature of the steam can be raised in the boiler the higher the overall electrical efficiency of the plant is, i.e. the ratio between the net production of electric power and the consumption of process heat. There is a need to raise the overall electrical efficiency of the soda recovery boilers nearer to that of the conventional coal-fired power plants, i.e., the pressure and the temperature of the steam produced by the soda recovery boilers should be raised to as high a level as possible. Today, the overall electrical efficiency of a soda recovery boiler is about 25%. It would be more advantageous to produce as much electricity by steam as possible because the possible overproduction of steam is more easy to utilize in the form of electricity.
In other industrial boilers, a conventional steam pressure/temperature is e.g. 130 bar/535.degree. C. In soda recovery boilers, the pressure and the temperature have to be regulated in accordance with the strength of the available pressure vessel steels. In the superheater of the soda recovery boilers, ferritic heat resistant steels and austenitic steels reach a longer working life in the hottest part only when the surface temperature of the tubes does not exceed 550.degree.-600.degree. C. The temperature of the superheated steam in the soda recovery boilers is therefore usually not allowed to rise to 500.degree. C. At a pressure of 60 -90 bar, a temperature of 450.degree.-480.degree. C. is usually considered to be the maximum temperature.
Attempts have been made to reach higher temperatures in the superheating sections of soda recovery boilers than mentioned above, for example by controlling the combustion process in the boiler. The incoming secondary and tertiary air of the boiler have been controlled in order to achieve as even a combustion process as possible, in which no great temperature variations in the flue gas flow occur. The purpose has been in this way to eliminate sudden and, as regards corrosion, dangerously high temperature peaks in the superheater section, in which case it would be possible to increase the mean temperature in the superheater section. In this way, an increase of a few degrees can possibly be achieved in the superheater section temperature.
Attempts have also been made to achieve higher steam temperatures without the risk of corrosion by reducing the fouling of the superheater surfaces. It is possible to some extent, for instance by appropriate feeding of the air, to reduce the amount of molten, inorganic material carried by the flue gas flow upwards to the superheater section to foul the surfaces of the superheater. On the other hand, the formation of deposits can also be reduced by continuous sweeping.
Attempts have also been made to reduce the fouling of the surfaces and the clogging of the flue gas channels by dimensioning the convection section sufficiently large and increasing the distance between the superheater surfaces. Larger clearences facilitate the sweeping and cleaning of the surfaces. These arrangements increase, however, the size of the boiler and are thus, as regards the building costs, unfavourable.
A soda recovery boiler in which the size of the superheater has had to be increased is, as regards the heat transfer and evaporation efficiency, inferior to a corresponding coal-fired boiler. The tendency of the superheater surfaces to foul multiplies the number of heat surfaces needed in comparison with a boiler, the flue gases of which contain only small amounts of ash or no ash at all.
The suggested improvements mentioned above have not proved to guarantee a continuous production of steam having a temperature of above 500.degree. C. The corrosion of the surfaces of the superheater proceeds in spite of the alterations, at an uneconomically fast rate. So far, no such method is being used in pulp mills, by means of which steam of the same high quality could be produced in soda recovery boiler plants as in other, conventional power plant boilers.